In the assembly of surface mount printed circuit boards, many dots of liquid epoxy or solder for connecting components to the circuit boards are dispensed on the circuit boards. These components can include discrete components, such as resistors and capacitors, and integrated circuit chips or chip holders. Commercial dispensing machines typically dispense thousands of dots of such liquid per hour, and are expected to run continuously to achieve high throughput.
Knight Tool Co., Inc., of Haverhill, Massachusetts, the assignee of the present invention, manufactures liquid dispensing systems that use a rotary positive displacement pump assembly. These systems are distributed by Camelot Systems, Inc., under the registered trademark CAM/ALOT.RTM.. In a typical dispensing system, a pump assembly is mounted to a moving assembly for moving the pump assembly along three mutually orthogonal axes, typically with three lead screws. To dispense a dot of liquid on a desired location, the moving assembly moves the pump assembly in a horizontal x-y plane and stops it over the desired location. The pump assembly is lowered with a z-axis lead screw along the vertical z-axis until the nozzle is at an appropriate height over the board. The pump assembly dispenses a dot of liquid, and is then raised along the z-axis, moved in the x-y plane to a next desired location, and lowered along the z-axis to dispense a next liquid dot.
A number of different approaches, have been used for dispensing small volumes of liquid as individual dots at a high rate. One approach is embodied in a pump manufactured by the assignee and known as a "Dual-Height Rotary Displacement Pump", described in "Design News", April 1994. In this type of pump, an electromagnetically operated clutch is selectively activated to couple a motor and a dispensing cartridge. The cartridge houses an augering screw in an auger chamber. The clutch has a top plate that is continuously rotated by the motor, and a bottom plate that is rotatably connected through intermediate coupling members, including a metal bellows, to the augering screw in the cartridge. The liquid to be dispensed is held in a vertical, cylindrical syringe, and is provided to the auger chamber under constant low pressure.
A controller selectively provides to the clutch a short, timed, electrical signal that induces magnetic attraction between the top and bottom plates. This attraction causes the plates to be engaged and to rotate together for a short period of time. The rotation by the bottom plate causes the screw to rotate a small amount, thus dispensing a small amount of liquid through a nozzle that is screwed to the cartridge.
The nozzle through which the liquid is dispensed is adjacent a mechanical sensing foot that contacts the medium on which the dot is to be dispensed to define a fixed z-axis displacement between the nozzle and the medium. As the pump assembly is lowered and the foot contacts the medium, the metal bellows is compressed as needed. The z-axis displacement is important because if it is incorrect, the dot can have an incorrect size and shape, and can exhibit one of a number of flaws, such as those known in the industry as tailing, stringing, or mushrooming. Such sensing is particularly important when dispensing on a warped medium.
While some pump models dispense at only one height, this pump model has a dual-height feature that allows the pump to dispense at one of two different heights. An air feed is provided to the cartridge to selectively raise and lower the foot relative to the nozzle and cartridge. When the distance between the end of the nozzle and the end of the foot is increased, a larger dot can be dispensed. This dual-height feature provides added flexibility, but it comes at the expense of some throughput.
Because circuit boards have a number of different types of components, it is often desirable to provide dots that have different volumes of liquid; different profiles, including different diameters and shapes, such as a circular or horseshoe shape; or different types of liquid. While the pump model described above can dispense dots at two different heights to provide different size dots, it can use only one nozzle at a time, and therefore cannot dispense different types of liquids or different shapes.
To accommodate multiple nozzles, there are liquid dispensing systems that have a number of pump assemblies arranged in a line and clamped together. One such system receives liquid from two liquid sources, and dispenses the liquid through two respective assemblies that include two respective motors. Another way that multiple dispensers have been provided is with a model that uses four separate air cylinders to dispense the liquid.
An object of the present invention is to improve the flexibility of a liquid dispensing system.
Another object of the present invention is to improve the throughput of a liquid dispensing system without sacrificing repeatability and accuracy.